1. Field of the Invention
The present invention relates to the packaging of semiconductor chips and particularly to that of micro-electrical mechanical systems (MEMS) such as micromirror devices.
2. Description of the Related Art
Packaging is a critical part of producing a high-performance MICROMIRROR for use in optical applications. Typically in a micromirror, the mirrors land on the substrate surface of the device. To avoid the mirrors from sticking, it is necessary that the micromirror be packaged in a controlled environment, with minimum amounts of moisture, adhesives, dust, and other contaminants. In order to provide such an environment, hermetically sealed packages are often used. However, the cost of such packages and the negative impact on the assembly process is extremely high.
The packaging of micromirror chips for use in projection display and other electro-optical applications has continued to present a cost barrier that contributes to higher prices for these products. A lower cost micromirror package is required to reduce the cost of these high-resolution, digital projectors. Today""s micromirror packages are mostly built on custom designed ceramic substrates and have expensive glass covers (lids), which are seam welded or fixed in place with an adhesive. These packages are not only expensive, but they require a low throughput process that reduces the product cycle time.
FIG. 1 is a drawing of a welded package. This is a hermetically sealed package that consists of a ceramic substrate 10 with a built-in Kovar lid-attaching ring 11 on its surface and a lid assembly, which consists of a Kovar frame 12 with built-in an optical quality glass window 13. In the assembly process the micromirror die 14 is attached to the substrate to provide both mechanical and thermal stability, as well as an electrical ground plane. The micromirror leads are bonded to pads on the substrate 10 which extend to external package pads on the edges or bottom of the package. The package is filled with an inert gas and then the lid assembly is seam welded at the two mating surfaces 15 between the lid frame 12 and lid-attaching ring 11.
The adhesive sealed package shown in FIG. 2 has been used in place of the welded package to lower the cost and improve the manufacturing throughput. This package is similar in that it has a ceramic substrate 20 but the cover glass 21 (lid) is a single piece of optical quality glass. In this case, the micromirror 22 is mounted and bonded out in the substrate""s 20 cavity and then the cover glass 21 is attached using an adhesive. The main advantages are that the assembly process is somewhat simplified, which improves the manufacturing throughput and the glass covers 21 are simple pieces of glass that can be sourced from various vendors.
The micromirror packages discussed above perform very well but are too expensive and tend to limit cost reduction efforts due to their high cost material and labor content. What is needed is a simple micromirror packaging approach that is low cost, easily assembled, and reliable. The invention disclosed herein fulfills this need.
This invention discloses a low cost, high performance, reliable micromirror package that overcomes many of the shortcomings of the conventional ceramic packages commonly used. This approach replaces the ceramic substrate with a printed circuit board substrate, the ceramic case with a molded plastic case, and the cover glass with an optical quality plastic window or lid. The printed circuit board substrate allows for either external bond pads or flex cable connection of the micromirror package to the projector""s motherboard. These packages support flexible snap-in, screw-in, ultrasonic plastic welding, or adhesive welding processes to overcome the low throughput, high cost, seam welding process of many conventional packages.
Other advantages include the following:
1. Requires no seam welding,
2. readily supports assembly automation,
3. uses multiple source, commodity piece parts,
4. light weight,
5. has built-in thermal plane on the bottom of the printed circuit board substrate,
6. supports standard chip connection methods, such as edge pad, grid-pad, or flex cable, and
7. lower cost.